This invention relates generally to a tree harvesting machine attachable to a vehicle and, more particularly, to a hydraulic circuit for controlling the functions of harvesting machines of the character shown and described in Moser et al. U.S. Pat. No. 3,842,872 entitled "Delimber Assembly For Tree Harvesting Machines" issued Oct. 22, 1974, Moser et al. U.S. Pat. No. 3,731,720 entitled "Shear Assembly For Tree Harvesters" issued May 8, 1973, and Gutman et al. U.S. Pat. No. 3,669,161 entitled "Drive Mechanism For Tree Harvesters" issued June 13, 1972. All of the above are assigned to the assignee of the present invention. A more complete description of the various components, their respective functions and their interrelationship is found therein.
A prior hydraulic control circuit used with a tree harvesting machine is shown in Moser U.S. Pat. No. 3,690,354 entitled "Tree Harvester Control Circuit" issued Sept. 12, 1972, and also assigned to the assignee of the present invention. Therein, a single control valve having two operating positions regulated substantially the entire tree harvesting operation. In addition, the pressures within the grapple arms and the hydraulic drive motor were interdependent so that as the load on the drive motor increased during delimbing, the grapple arm pressure increased to insure that the tree remained effectively engaged with the drive mechanism. Similarly, when the drive motor load decreased, the grapple arm pressure decreased.